Stepping motor

ABSTRACT

A stepping motor has a relationship between an outer diameter of a rotor and an outer diameter of a bearing made of magnetic material that can be adjusted appropriately so as to impart magnetic attractive force to the bearing, the stepping motor including a rotor  400  made of a permanent magnet, a stator  500  including multiple pole teeth  223, 243, 323  and  343  extending in an axial direction of the rotor arranged at an outer circumferential side of the rotor  400,  a bearing  250  rotatably supporting one end portion in the axial direction of the rotor  400,  and a bearing  260  rotatably supporting the other end portion in the axial direction of the rotor  400.  Magnetic permeability of the bearing  260  is greater than that of the bearing  250,  the rotor  400  includes multiple magnetic poles on an outer circumferential surface along a circumferential direction of the rotor  400,  and an outer diameter D R  of the rotor  400,  an outer diameter D B  of a portion of the bearing  260  which is facing the rotor  400,  and an inner diameter D SI  of the stator  500  are set to satisfy the following relationship of formula 1 
       D R &lt;D B &lt;D SI   (formula 1)

TECHNICAL FIELD

The present invention relates to stepping motors, and in particular,relates to PM-type (Permanent Magnet-type) stepping motors having arotor which is a permanent magnet and a stator having multiple poleteeth extending in an axial direction at an outer circumferential sideof the rotor.

BACKGROUND ART

Conventionally, in the abovementioned PM-type stepping motor, a bearingrotatably supporting the rotor is installed, and there is a gap alongthe axial direction between the rotor and the bearing. For example, in atwo-phase PM-type stepping motor, there is a phenomenon due tocombination of excitation of two stators, in which the magnetic centersof the rotor and the stator are misaligned during driving of thestepping motor, and the rotor moves along an axial direction, therebycausing abnormal noise by collision with the bearing.

To solve this phenomenon, a stepping motor is known in which a magneticattractive force generating member attracting a rotor in the axialdirection is arranged, and the rotor is attracted to one bearing,thereby reducing striking noise by collision with the other bearing (seePatent Document 1)

The stepping motor disclosed in Patent Document 1 is a two-phase PM-typestepping motor including a rotor magnet which is a permanent magnet anda stator having multiple pole teeth extending in an axial direction at acircumferential side of the rotor, and having a structure in which amagnetic washer as the magnetic attractive force generating member madeof magnetic material affixed to a case plate metal fixing the bearing.In this stepping motor, outer diameter D_(M1) of the magnetic washer andouter diameter D_(R) of the rotor magnet are set to satisfy arelationship D_(M1)=<D_(R) (see claim 3). In addition, the rotor magnetis attracted to the magnetic washer side so as to reduce striking noiseduring rotation driving of the stepping motor.

Patent Document 1 is Japanese Unexamined Patent Application PublicationNo. 2014-3832

SUMMARY OF THE INVENTION

However, in a case in which the stepping motor is used in an electronicdevice, such as a mobile phone, such as a smart phone, an outer diameterof the stepping motor is not more than 6 mm, and an outer diameter ofthe rotor of such a stepping motor is very small, being not more than 3mm. In such a case of a very small diameter motor, it is difficult toobtain a space to arrange the magnetic washer of the Patent Document 1.Therefore, it can be considered that the bearing itself is formed by amagnetic material instead of the magnetic washer.

On the other hand, the Patent Document 1 also discloses that instead ofthe magnetic washer and the bearing, a bearing made of a magneticmaterial and enlarged in a radial direction can be used as the magneticattractive force generating member. (see [0041]). In that case, since itis assumed that an outer diameter of the bearing made of a magneticmaterial and enlarged in a radial direction is similar to an outerdiameter D_(M1) of the magnetic washer, D_(M1) (outer diameter of thebearing) and D_(R) (outer diameter of the rotor magnet) are set so as tosatisfy the relationship D_(M1)=<D_(R).

However, in Patent Document 1, the rotor magnet is magnetized at thebottom surface thereof facing the magnetic washer in addition to theside surface thereof facing the excitation stator. Therefore, it isunclear whether or not the abovementioned effect can be obtained even ina case in which the bottom surface of the rotor magnet is notmagnetized.

The present invention has been completed in view of the abovecircumstances, and an object of the present invention is to provide astepping motor in which a relationship between the outer diameter of arotor and the outer diameter of a bearing made of magnetic material areappropriately adjusted in a case in which the bearing made of magneticmaterial is used as a magnetic attractive force generating memberinstead of a magnetic washer and a bearing.

The present invention is a stepping motor including: a rotor made of apermanent magnet, a stator including multiple pole teeth extending in anaxial direction of the rotor arranged at an outer circumferential sideof the rotor, a primary bearing rotatably supporting one end portion inthe axial direction of the rotor, and a secondary bearing rotatablysupporting the other end portion in the axial direction of the rotor, inwhich magnetic permeability of the secondary bearing is greater thanthat of the primary bearing, the rotor includes multiple magnetic poleson an outer circumferential surface along a circumferential direction ofthe rotor, and an outer diameter D_(R) of the rotor, an outer diameterD_(B) of a portion of the secondary bearing which is facing the rotor,and an inner diameter D_(SI) of the stator are set to satisfy thefollowing relationship of Formula 1.

D_(R)<D_(B)<D_(SI)  (formula 1)

In the present invention, since magnetic permeability of the secondarybearing is set higher than that of the primary bearing, magnetic force(magnetic flux) of the rotor acts more strongly at the secondary bearingthan at the primary bearing, and magnetic attractive force to the rotoris generated at the secondary bearing. Here, since the outer diameterD_(B) of the portion of the secondary bearing facing the rotor is setgreater than the outer diameter D_(R) of the rotor, a part of magneticforce (magnetic flux) at magnetic poles (N pole and S pole) formed on anouter circumferential surface of the rotor in addition to magnetic force(magnetic flux) leaked from the end surface of the rotor affects thesecondary bearing over the end surface of the rotor, and the secondarybearing acts as a magnetic attractive force generating member.Therefore, the rotor is attracted to the secondary bearing, therebyreducing the striking noise occurring during rotation driving of thestepping motor.

In the present invention, action and effect based on Formula 1 can beobtained only in a case in which the magnetic poles are arranged only onthe outer circumferential surface of the rotor. However, in a case inwhich the magnetic poles are arranged on an end surface of the rotor,since greater magnetic force acts at the secondary bearing, the presentinvention does not exclude such an embodiment.

In the present invention, it is desirable that the secondary bearinginclude a small diameter portion and a large diameter portion, the smalldiameter portion be affixed to a covering member, and the large diameterportion face an end surface of the rotor. In this way, the size of thesecondary bearing can be reduced, thereby reducing material cost, andthe secondary bearing can be attached to the covering member via a stepportion between the small diameter portion and the large diameterportion.

In the present invention, by arranging a resin washer between the rotorand the secondary bearing, metals of the rotor and the secondary bearingare prevented from making contact with each other, thereby avoiding theoccurrence of abnormal noise.

According to the present invention, the stepping motor in which therelationship between the outer diameter of a rotor and the outerdiameter of a bearing made of magnetic material is adjustedappropriately can be provided in a case in which the bearing made ofmagnetic material is used as a magnetic attractive force generatingmember instead of a magnetic washer and a bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken exploded perspective view of the steppingmotor of the embodiment of the present invention.

FIG. 2 is a side view of the stepping motor of the embodiment of thepresent invention.

FIG. 3 is a side cross sectional view of the stepping motor of theembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION 1. Overall Structure ofStepping Motor

FIG. 1 shows a stepping motor 100 of the embodiment. The stepping motor100 is a claw-pole-type two-phase PM-type (Permanent Magnet-type)stepping motor. The stepping motor 100 includes a stator 500. The stator500 has a structure in which an A phase stator unit 200 and a B phasestator unit 300 are combined in an axial direction. Here, the A phasestator unit 200 and the B phase stator unit 300 have the same structure,and one is opposite to the other in an axial direction, back surfacesthereof are in contact and are combined, thereby constructing the stator500.

A front plate 210 is fixed to the A phase stator unit 200, and an endplate 310 is fixed to the B phase stator unit 300. The stator 500 has asubstantially cylindrical shape, and a rotor 400 is contained therein ina rotatable condition.

The A phase stator unit 200 has a structure in which an outer yoke 220,a bobbin 230, and an inner yoke 240 are combined in an axial direction.The outer yoke 220 is a portion which functions as a yoke at which amagnetic path is formed, and is made of a magnetic material such aselectromagnetic soft iron or rolled steel plate. The outer yoke 220 hasa shape which is substantially cup-shaped, the central portion of whichis open, and has substantially triangular poles 223 which extend from anend portion of the opening portion in an axial direction and which aremultiply arranged along a circumferential direction with a certain gaptherebetween.

A coil 231 (shown only in FIG. 3) is wound around the bobbin 230. Thebobbin 230 is made of resin and is formed by an injection moldingmethod. Flange portions 233 and 234 are formed at both ends along anaxial direction of the bobbin 230 in order to prevent releasing of thewinding of the coil 231. At one flange portion 234, a terminal block 235is formed. The terminal block 235 is a rectangular block which protrudesfrom the flange portion 234 in an axial direction and in a radialdirection and is integrally formed with the flange portion 234. Multiple(in this embodiment, two) metallic terminal pins 236 are disposed in theterminal block 235 by a method such as press fitting or insert molding.

The bobbin 230 having the abovementioned structure is arrangedsandwiched by the outer yoke 220 and the inner yoke 240, therebyconstructing the A phase stator unit 200. Furthermore, a bobbin 330 ofthe B phase stator unit 300 is also similarly arranged sandwiched by anouter yoke 320 and an inner yoke 340, thereby constructing the B phasestator unit 300.

The inner yoke 240 is a portion which functions as a yoke at which amagnetic path is formed, and is made of a magnetic material such aselectromagnetic soft iron or rolled steel plate. The inner yoke 240 hasa tabular circular ring portion 241 and substantially triangular poleteeth 243 which extend from an end portion of an inner circumferentialside of the circular ring portion 241 to an axial direction and whichare multiply arranged along a circumferential direction with a certaingap therebetween. The pole teeth 243 of the inner yoke 240 and poleteeth 223 of the outer yoke 220 are intermeshed with each other in acircumferential direction.

The inner yoke 340 of the B phase stator unit 300 is one which isreversing a member similar to the inner yoke 240 of the A phase statorunit 200 along an axial direction. By facing and contacting the samesurfaces of the inner yoke 240 and the inner yoke 340, the A phasestator unit 200 and the B phase stator unit 300 are combined.

The front plate 210 is fixed with a surface of the outer yoke 220opposite to the surface facing the bobbin 230. A bearing (primarybearing) 250 is attached to the front plate 210.

The B phase stator unit 300 has almost the same structure as that of theA phase stator unit 200, and one having almost the same structure as theA phase stator unit 200 is used in a reversed condition along an axialdirection as the B phase stator unit 300. The B phase stator unit 300has a structure in which an outer yoke 320, a bobbin 330, and an inneryoke 340 are combined in an axial direction. Here, the outer yoke 320 isa part having almost the same structure as the outer yoke 220, andincludes substantially triangular pole teeth 323. The inner yoke 340 isa part having almost the same structure as the inner yoke 240, andincludes substantially triangular pole teeth 343. The bobbin 330 is apart having almost the same structure as the bobbin 230. The end plate310 is fixed with the outer yoke 320. A bearing (secondary bearing) 260is attached to the end plate 310.

2. Details of Rotor and Supporting Structure Thereof

A shaft 401 is fixed to the axial center of the rotor 400 in a conditionpenetrating in an axial direction, one end portion of the shaft 401protruding from the rotor 400 to the left side in FIG. 3 is rotatablysupported by the front plate 210 via the bearing 250, and the other endportion of the shaft 401 protruding from the rotor 400 to the right sidein FIG. 3 is rotatably supported by the end plate 310 via the bearing260.

The bearing 250 of the A phase stator unit 200 is constructed by acopper-based oil-impregnated sintered bearing for example, and as shownin FIG. 3, has a two-step structure having a large diameter portion 251and a small diameter portion 252, and is affixed by engaging the smalldiameter portion 252 into an opening of the front plate 210.

On the other hand, the bearing 260 of the B phase stator unit 300 isformed by a magnetic material (for example, an iron-copper-basedoil-impregnated sintered bearing or an iron-based oil-impregnatedsintered bearing), has a two-step structure having a large diameterportion 261 and a small diameter portion 262, and is affixed by engagingthe small diameter portion 262 into an opening of the front plate 310.In addition, magnetic permeability of the bearing 260 is set higher thanthat of the bearing 250.

A resin washer 402 is attached on an end surface of the rotor 400 facingthe bearing 250 side. Furthermore, a resin washer 403 is attached on anend surface of the rotor 400 facing the bearing 260 side. Therelationship between the outer diameter D_(B) of the large diameterportion 261 of the bearing 260 and the outer diameter D_(R) of the rotor400 is set as D_(B)=<D_(R).

The rotor 400 is made of a permanent magnet such as a ferrite magnet, arare earth magnet, or the like, and has a substantially cylindricalstructure. This permanent magnet has a magnetic pole which is magnetizedin a condition in which the magnetic pole alternates as NSNS along thecircumferential direction.

Here, in order for the bearing 260 to function as a magnetic attractiveforce generating member, it is necessary that magnetic force (magneticflux) of the rotor 400 affect the bearing 260. Therefore, the outerdiameter D_(B) of the large diameter portion 261 of the bearing 260facing the end surface of the rotor 400 and the outer diameter D_(R) ofthe rotor are set to be relationship D_(R)<D_(B). In addition, in orderto prevent interference of the large diameter portion 261 and the stator500, the outer diameter D_(B) of the large diameter portion 261 and theinner diameter D_(SI) of the stator 500 are set to be relationshipD_(B)<D_(SI).

Since the outer diameter D_(B) of the large diameter portion 261 of thebearing 260 is set with a relationship D_(R)<D_(B), a part of magneticforce (magnetic flux) at magnetic poles (N pole and S pole) formed onthe outer circumferential surface of the rotor 400 in addition tomagnetic force (magnetic flux) leaked from the end surface of the rotor400 affect the secondary bearing 260 over the end surface of the rotor400, and the bearing acts as the magnetic attractive force generatingmember. Therefore, the rotor 400 is attracted to the bearing 260 side,and the end surface of the rotor 400 contacts the resin washer 403.

In a case in which exciting current is supplied on the coils 231 and 331wound around the bobbins 230 and 330, the rotor 400 rotates by aninteraction between magnetic flux excited in the stator 500 and magneticflux of the rotor 400 which is a permanent magnet. According to acombination of an excitation method of the stator 500, when only the Aphase stator unit 200 is excited (one phase excitation), althoughmagnetic centers of the rotor 400 and the stator 500 are misaligned andthereby generates a force which moves the rotor 400 to the bearing 250side, the movement of the rotor 400 in an axial direction is restrainedby magnetic attractive force generated by the bearing 260 which is madeof magnetic material. Therefore, the striking noise generated duringrotation driving of the stepping motor 100 can be reduced.

It should be noted that movement of the rotor 400 along an axialdirection can be blocked by setting magnetic attractive force actingbetween the bearing 260 made of magnetic material and the rotor 400 tobe greater than magnetic force acting in an axial direction of the rotor400 (direction of bearing 250). Such setting can be achieved, forexample, by adjusting thickness (size in an axial direction) of theresin washer 403 arranged between the bearing 260 and the rotor 400, orby adjusting magnetic permeability of the bearing 260.

In particular, in the above embodiment, size of the bearing 260 can bereduced so as to reduce material cost, and the step portion between thesmall diameter portion 262 and the large diameter portion 261 can beattached to the end plate 310, since the bearing 260 has the smalldiameter portion 262 and the large diameter portion 261, the smalldiameter portion 262 is affixed to the end plate 310, and the largediameter portion 261 faces to an end surface of the rotor 400.

In the above embodiment, since the resin washer 403 is arranged betweenthe rotor 400 and the bearing 260, metals of the rotor 400 and thebearing 260 can be prevented from making contact, thereby preventingoccurrence of abnormal noise.

3. Modifications

The present invention is not limited to only a claw-pole-type two-phasestepping motor as mentioned above in the embodiments, and it can beapplied to other multiphase PM-type stepping motors.

The present invention can be applied to the technical field of steppingmotors.

EXPLANATION OF REFERENCE NUMERALS

100: Stepping motor, 200: A phase stator unit, 210: front plate, 220:outer yoke, 223: pole tooth, 230: bobbin, 231: coil, 233: flangeportion, 234: flange portion, 235: terminal block, 236: terminal pin,240: inner yoke, 241: circular ring portion, 243: pole tooth, 250:bearing (primary bearing), 251: large diameter portion, 252: smalldiameter portion, 260: bearing (secondary bearing), 261: large diameterportion, 262: small diameter portion, 300: B phase stator unit, 310: endplate, 320: outer yoke, 323: pole tooth, 330: bobbin, 331: coil, 340:inner yoke, 343: pole tooth, 400: rotor, 401: shaft, 402: resin washer,403: resin washer, 500: stator.

What is claimed is:
 1. A stepping motor comprising: a rotor made of apermanent magnet, a stator including multiple pole teeth extending in anaxial direction of the rotor arranged at an outer circumferential sideof the rotor, a primary bearing rotatably supporting one end portion inthe axial direction of the rotor, and a secondary bearing rotatablysupporting the other end portion in the axial direction of the rotor,wherein magnetic permeability of the secondary bearing is higher thanthat of the primary bearing, the rotor includes multiple magnetic poleson an outer circumferential surface along a circumferential direction ofthe rotor, and an outer diameter D_(R) of the rotor, an outer diameterD_(B) of a portion of the secondary bearing which is facing the rotor,and an inner diameter D_(SI) of the stator are set to satisfy thefollowing relationship:D_(R)<D_(B)<D_(SI)  (formula 1)
 2. The stepping motor according to claim1, wherein the magnetic poles are arranged only on the outercircumferential surface of the rotor.
 3. The stepping motor according toclaim 1, wherein the secondary bearing includes a small diameter portionand a large diameter portion, the small diameter portion is affixed to acovering member, and the large diameter portion faces an end surface ofthe rotor.
 4. The stepping motor according to claim 2, wherein thesecondary bearing includes a small diameter portion and a large diameterportion, the small diameter portion is affixed to a covering member, andthe large diameter portion faces an end surface of the rotor.
 5. Thestepping motor according to claim 1, wherein a resin washer is arrangedbetween the rotor and the secondary bearing.
 6. The stepping motoraccording to claim 2, wherein a resin washer is arranged between therotor and the secondary bearing.
 7. The stepping motor according toclaim 3, wherein a resin washer is arranged between the rotor and thesecondary bearing.
 8. The stepping motor according to claim 4, wherein aresin washer is arranged between the rotor and the secondary bearing.